In diesel engines used as automobile engines or the like, it is common practice to control the form of combustion, which takes place in a combustion chamber (in a cylinder), by adjusting the timing of fuel injection or the amount of fuel injection from a fuel injection valve (hereinafter also referred to as an injector) according to engine revolution, amount of accelerator operation, coolant temperature, intake air temperature, and the like.
Diesel engine combustion, as disclosed in patent document 1 below, is mainly includes premixed combustion and diffusion combustion. When fuel injection starts from the injector into the combustion chamber, the fuel is first vaporized and diffused to generate a combustible mixture (ignition delay period). Next, the combustible mixture ignites by itself approximately simultaneously at several positions in the combustion chamber; thus the combustion rapidly progresses (premixed combustion). The fuel injection into the combustion chamber continues so as to continue the combustion (diffusion combustion). Since unburned fuel exists even after the fuel injection ends, heating continues for some period of time (afterburning period).
In some engines, such as diesel engines, that involve lean combustion, a dominating part of the entire operating area is dedicated to combustion of mixtures of high air-fuel ratios (lean atmospheres). This raises a concern over relatively large amounts of nitrogen oxide (hereinafter referred to as NOx) emissions. Additionally, if a combustion of a mixture in the combustion chamber results in an incomplete combustion, smoke occurs in the exhaust gas, to the further detriment of exhaust emissions.
To reduce the amount of NOx generation, exhaust gas recirculation apparatuses (EGR: Exhaust Gas Recirculation) are known to recirculate part of the exhaust gas back into the intake path (see, for example, patent document 2). Also to improve exhaust emissions, it is known practice to carry out an auxiliary injection in the compression process of the engine so as to cause a premixed combustion using the auxiliary injection (see, for example, patent document 3 and patent document 4).